California Avocado Society 1956 Yearbook 40: 165-168
GROWTH OF AVOCADO FRUIT TISSUE ON ARTIFICIAL MEDIA
C. A. Schroeder
Professor, Department of Subtropical Horticulture
University of California, Los Angeles.
Investigations on the development and physiology of the avocado fruit have indicated
that it is unique compared with such temperate zone fruits as apples, peaches, or pears.
The latter fruits have been studied extensively and a knowledge has been acquired
concerning their chemical composition, anatomical development, and physiological
behavior on the tree and under post-harvest conditions. Studies on the physiology of the
avocado fruit by Biale (1) have shown that the avocado is characterized by an extremely
high rate of evolution of carbon dioxide when measured against a given weight of fruit
for a given time. The respiration rate of 150 to 180 mg. of CO2 per kilogram of fruit per
hour is among the highest recorded for important fruits. Another aspect of fruit
development markedly exhibited by the avocado is the sudden increase in respiration
activity for a short period and then a rapid decline, a phenomenon known as the
climacteric. This climacteric point in the avocado has been demonstrated by Biale to be
coincidental with the softening of the fruit.
Morphological and anatomical studies of the avocado fruit (3) have suggested that this
fruit differs substantially in its type of development compared with deciduous fruits. The
latter fruits, as a group, are characterized essentially by the following mode of
development. As a consequence of pollination and fecundation of the ovules, the ovary
is stimulated to undergo a period of cell multiplication, during which time cell divisions
occur and cell numbers increase in rapid succession. This period of cell division
continues for about three weeks, then ceases. Then follows a period during which
expansion and enlargement of the previously formed cells occurs. Cell enlargement
continues until the fruit is mature or harvested.
The avocado appears to be unique in respect to its developmental histology in the
following manner. Pollination and subsequent fecundation of the ovule stimulates the
avocado ovary to undergo cell division. Such divisions do not cease after the first
month, but instead continue as long as the fruit remains on the tree. Evidence that cell
divisions occur in nearly mature or mature fruits may readily be observed in properly
prepared materials (Plate 1).While the avocado fruit develops from an ovary about 1/16
inch in diameter to a mature size of four to five inches in diameter, the individual cells
reach a maximum size of approximately 0.0024 inch in diameter in the Fuerte variety, at
which time they undergo division to form two new cells. Because the maximum size of
cells appears to be approximately constant, the difference in size between fruits of a
given variety is determined by differences in number of cells. Hence, the larger the fruit
the greater the number of cells in that fruit. A calculation, based on a single sample,
indicated that a Fuerte fruit of 7 oz. contains approximately 1,350,000,000 cells.
Because the avocado fruit is characterized by this ability to undergo cell division at all
stages, even as it approaches maturity, an attempt was made to maintain this state of
cell multiplication in pieces of tissue separated from the fruit. Slices of tissue 8 mm. in
diameter and 2 mm. thick were taken under sterile conditions from nearly mature fruit
and placed in glass vials on an agar media (Figure 1). The culture media contained the
basic ingredient; required for the nutrition of plants in general, such as nitrate, sulphate,
magnesium, calcium, etc., and in addition there were added such materials as vitamin
B6, growth promoting substances such as indoleacetic acid, amino acids of various
types, and coconut milk.
Such tissue slices, placed in vials and kept at room temperature, may begin to show
indications of growth in three weeks. The first evidence that the sections are alive is the
appearance of cellular proliferation on the upper surface of the tissue disk. This surface
tuft results from divisions of cells at the surface and the uplifting of these algal-like
strings of cells (Figures 2 & 3). In some tissue disks no external evidence of growth can
be detected, but instead internal cell division may be observed upon sectioning and
examination under the microscope. This activity does not result in immediate expansion
or growth of the tissue. Cambial-like layers are occasionally found, the activity of which
results in a large number of small brick-shaped cells which are lifted en masse, such
that they cause the disk to increase in thickness and diameter.
The investigations now under way are directed toward the objective of learning the
basic requirements of such excised tissue disks. These requirements can only be
determined by extensive trials utilizing various media of different composition and
subjecting the materials to various conditions of light, temperature, aeration, etc. The
goal of all the studies is to maintain the tissues on a completely artificial media for an
indefinite time so that effects of specific factors, such as beat or light, or a given mineral
element can be determined. A knowledge of these basic requirements will be helpful in
understanding and more intelligently pursuing the problems of fruit development
encountered in the field and in handling processes.
These attempts to "tissue culture" explants from mature fruit tissues appear to be the
first to result in success, as judged by reports in the literature (3). Previous attempts (4)
to culture apple and other deciduous fruit tissues have given negative results, probably
because of the fact that such-tissues lack the potentiality to undergo cell division as the
fruit approaches maturity, or perhaps success might be attained with other techniques.
In the present experiments, tissue sections of avocado fruit have been maintained
successfully in living conditions and showing evidence of cell division for periods greater
than 5 months. These studies will be continued to determine the ultimate life length to
learn of the basic requirements for growth of the tissues.
LITERATURE CITED
1. Biale, J. B. The climacteric rise in respiration rate of the Fuerte avocado fruit. Proc.
Amer. Soc. Hort. Sci. 39: 137-142. 1941.
2. Caplin, S., M., and Stewart, F. C. Investigations on growth and metabolism of plant
cells. III. Variables affecting the growth of tissue explants and the development of a
quantitative method using carrot root. Ann. Bot. 16: 211-231. 1952.
3. Schroeder, C. A. Growth and development of the Fuerte avocado fruit. Proc. Amer.
Soc. Hort. Sci. 61: 103-109. 1953.
4. Schroeder, C. A. Proliferation of mature fruit pericarp tissue slices in vitro. Science
122: 601. 1955.